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United States Patent 0 ice
3,089,856
Patented May 14, 1963
2
1
form of a spray.
3,089,856
PRODUCTION OF CONDUCTIVE ZINC OXIDE
Howard M. Cyr and Nicholas S. Nanovic, Palmerton,
Pa., assignors to The New Jersey Zinc Company, New
York, N.Y., a corporation of New Jersey
No Drawing. Filed Nov. 10, 1960, Ser. No. 68,332
2 Claims. (Cl. 252-518)
Greater uniformity of dissemination
of the activator in the oxide is obtained by slurrying the
oxide in water and by adding the activator either in the
solid form or in solution form. When the oxide is
slurried for addition of the activator, the ?nal mixture is
?ltered, washed and dried before being further treated.
The effective amounts of activator are generally within
the range of about 0.1 to 1% of the metallic constituent
of the activator by weight of the zinc oxide, and are pref
This invention relates to the production of electrically
conductive zinc oxide. More particularly, it comprises 10 erably within the range of 0.3 to 0.5%. Within this
preferred range, the conductivity of the ?nal product is
an improvement in a previously developed method of
substantially uniform, and variations from this degree of
converting normally non-conductive zinc oxide to a con
ductive form.
Electrically conductive zinc oxide is required as a coat
conductivity are not signi?cant until the amount of activa
tor is below or above the generally e?ective range of
ing for paper used in electro-photographic reproduction, 15 0.1 to 1%. However, the uniformity of application of
the activator to the oxide does affect the utilization of
and the United States patent to Dalton No. 2,887,632
the activator with the result that smaller amounts of
describes a representative procedure for converting the
activator are more effective when they are more uniformly
normally non-conductive oxide to a conductive form.
distributed through the zinc oxide than larger amounts
This procedure involves the addition of an activating
of
activator which are less uniformly distributed.
20
agent to the non-conductive oxide and the subsequent
The activator-containing zinc oxide is heated in the
heating of the thus-treated oxide in a reducing atmos
ambient atmosphere to an oxide temperature of at least
phere. Although the final product has the desired elec
about 650° C. The length of time reuqired for this heat
trical conductivity, the heat treatment results in oxide
ing
depends merely on the apparatus used for this pur
particle size growth such as to detract from the normal
attributes of zinc oxide as a coating pigment. Mechani 25 pose, the only requirement being that the entire mass
of oxide be raised to the aforementioned temperature.
cal disintegration of the heat-treated conductive oxide to
In order to insure uniform heating to this extent, we have
overcome the adverse e?‘ects of particle size growth and
agglomeration has simultaneously lowered its conduc
tivity.
found it advisable to heat the oxide mass to a measured
temperature of 700° C. Heating to 800° C. is no more
We have now discovered that the pigment Jbene?ca 30 elfective than 700° C. but may be used if desired to
insure more complete removal of volatile components
tion of mechanical disintegration can be realized in such
of: the activator compound. These volatile components
a method of imparting electrical conductivity to zinc
are, for example, zinc chloride when the activator is
oxide if a major portion of the heat treatment of the
‘aluminum chloride, nitrous oxide when the activator is
activator-containing oxide is ?rst carried out in the am
a nitrate, sulfur dioxide when the activator is a sulfate,
bient atmosphere and if mechanical disintegration fol
carbon dioxide when the activator is an acetate or for
lows this heat treatment but precedes the ultimate re
mate, etc. The removal of these volatile materials ap
quisite heating of the oxide in a reducing atmosphere.
pears to improve the elfectiveness of the ?nal activation
Thus, our invention comprises an improvement in the
method of producing electrically conductive zinc oxide
wherein an activating agent is added to normally non
conductive zinc oxide and the activator-containing oxide
is heated in a reducing atmosphere to a temperature of at
least about 800° C. with resulting increase in its conduc
tivity. Our improvement in this method comprises ?rst
heating the activator-containing non-conductive zinc oxide
operation and simpli?es this operation which. unlike the
air-heating step, must be carried out in a closed vessel
in order to maintain the necessary reducing atmosphere.
In addition, the air-heating of the activator-containing
oxide appears to increase the uniformity of dissemination
of activator throughout the oxide and thus contributes
treatment.
An additional advantageous eifect of the step of heat
ing
the activator-containing zinc oxide in air prior to
about 650° C., subjecting this air-heated oxide to mechan
the activation heat-treatment in a reducing atmosphere is
ical disintegration wherein any agglomerates are broken,
and thereafter subjecting the disintegrated oxide to the 50 that most of the grain growth and agglomeration which
takes place during the course of the ultimate activation
aforementioned heat treatment in the reducing atmos
of the oxide appears to take place during the air-heating
phere.
step. These agglomerates and some enlarged grains can
The normally non-conductive zinc oxides which respond
be broken up by mechanical disintegration at the end or
to the method of our invention include both American
Process oxide, made directly from zinc ore, and French 55 the air-heating step without adversely affecting the degree
of conductivity ultimately obtained by the activated oxide.
Process oxide, made directly from zinc metal.
On the other hand, we have found that similar mechani
The activators which are useful in practicing our in
cal disintegration following the activation heat-treatment
vention are those already known for their ability to im
in the reducing atmosphere markedly lowers the conduc
part electrical conductivity to zinc oxide when the activa
tor-containing oxide is heated in a reducing atmosphere. 00 tivity of the oxide.
The mechanical disintegration of the air-heated activa
These activators generally comprise aluminum and indium
in the ambient atmosphere to a temperature of at least
metallo-organic and metallo-inorganic compounds such
tor-containing zinc oxide may be carried out in any con
ventional apparatus. Such a device is required only to
break up agglomerates; a signi?cant amount of grinding
of the pigment in such a device is neither required nor
as the acetates, formates, sulphates, nitrates, chlorides,
and the like. All such activators, it effective in the
method of the prior art, are at least equally effective, and 65 desired. Thus, we have found that excellent results are
in many instances more effective, when used in the
method of our invention.
obtained with a hammermill or ?uid energy mill which
The activator may be applied to the zinc oxide by any
procedure which will result in ettective distribution of
er than by grinding action.
the activator throughout the mass of oxide.
disintcgrates the agglomerates by mechanical impact rath
The ?nal activation heating must be done in a reducing
For ex 70 atmosphere. A mixture of 5% hydrogen with 95% nitro
ample, we have found that low cost and simplicity char
acterize application of a solution of the activator in the
gen is currently used as it is effective without being dan
3,089,856
3
4
gerously explosive if mixed with air, but concentrations
essed except that it was not passed through a hammermill,
the electrical resistivity of the product was 3.6)(105 ohm
up to 100% hydrogen have been used with satisfactory
results.
Other reducing gases such as carbon monoxide
cm. and therefore was not satisfactory as a conductive
and the products of incomplete combustion of propane
have also given good results.
The temperature of the reducing gas activation treat
ment controls the conductivity of the ?nal pigment to
some extent, with the highest temperatures giving best
oxide.
Example III
When the product from Example II was passed through
the hammermill after the reducing gas treatment, its elec
trical resistivity was further increased to 1.2x l06 ohm-cm.
results. However, satisfactory conductivity results from
an 800° C. heat treatment, whereas higher temperatures
up to about 1000” C. although effective nevertheless cause
Example IV
Sixty pounds of French Process pigment zinc oxide
some aggregation and undesirable particle size growth.
The length of this heat treatment is not per se important;
the only time requirement is that it ‘be sufficient to raise
were sprayed with a solution of 728 grams of AlCl3.6H2O
dissolved in 900 cc. of water. Since this salt contained
the entire mass of oxide to a temperature of about 800° C. 15 11.2% aluminum, the treatment added 0.3% aluminum
It is advantageous, however, to maintain the heat treat
ment for a su?icient period of time to insure uniformity of
activation of the oxide, all as is well understood in this
art. Following completion of this activation treatment,
to the weight of the zinc oxide. The activator-sprayed
oxide was then heated in air to 700° C., was cooled to
facilitate handling, and was then passed through the ham
mermill. The resulting disintegrated oxide was heated
the hot oxide should be cooled to at least 200° C., and 20 in a stainless steel retort in an atmosphere composed of
preferably to about 100° C., in the reducing or other non
5% hydrogen and 95% nitrogen by volume at a tempera
oxidizing atmosphere, before exposing it to the ambient
ture of 800° C. for one hour and was cooled to about
atmosphere.
room temperature in this atmosphere. The ?nal product
had an electrical resistivity of 2.7x l03 ohm-cm. and was
of our invention and show the effectiveness of not only 25 therefore a satisfactorily conductive zinc oxide.
the air-heating step but of the mechanical disintegration
We claim:
step which follows the air-heating but precedes the activa
1. In the method of producing electrically conductive
tion-heating operation. In each of these tests, the con
zinc oxide wherein an activating agent comprising alumi
ductivity of the treated oxide was determined by a dry
num chloride is added to normally non-conductive zinc
The following examples are illustrative of the practice
powder test (DPR) in which a plug of dry zinc oxide was 30 oxide and the activator-containing oxide is heated in a
compressed at 2600 p.s.i. in a glass tube having a bore
reducing atmosphere to a temperature of at least about
area of 0.0452 square inch. The length of the plug was
800° C. with resulting increase in its conductivity the
measured and its electrical resistance was also measured
improvement which comprises ?rst heating the activator
at a potential of 1 volt while maintaining the plug under
containing non-conductive zinc oxide in the ambient at
a compression pressure of 2000 psi. at room tempera 35 mosphere to a temperature of at least about 650° C., sub
ture. In this test, untreated zinc oxide (i.e. normally non
jecting this air-heated oxide to mechanical disintegration
conductive oxide) exhibits an electrical resistivity of 10'7
wherein any agglomerates are broken, and thereafter sub
to 109 ohm-centimeter. A resistivity of 104 ohm-centi
jecting the disintegrated oxide to the aforementioned heat
meter is considered a satisfactory indication of the elec
treatment in the reducing atmosphere.
trical conductivity of the oxide.
40
Example I
Twenty-?ve pounds of French Process pigment zinc
oxide were sprayed with a solution containing 505 grams
2. In the method of producing electrically conductive
zinc oxide wherein an activating agent comprising alumi
num chloride is added to normally non-conductive zinc
oxide and the activator-containing oxide is heated in a
reducing atmosphere to a temperature of at least about
of AlC]3.6H2O dissolved in 0.5 liter of water. This 45
800° C. with resulting increase in its conductivity, the im~
amount of aluminum chloride activator corresponds to
provement
which comprises ?rst heating the activator
0.5% aluminum based on the weight of the zinc oxide.
containing non-conductive zinc oxide in the ambient at
The activator-containing pigment was heated to 650° to
mosphere to a temperature of about 700° C., subjecting
700° C. in air, was cooled and was mechanically disin
tegrated by passing it through a hammermill. This dis 50 this lair-heated oxide to mechanical disintegration wherein
any agglomerates are broken, and thereafter subjecting
integrated product was heated to a temperature of about
the disintegrated oxide to the aforementioned heat treat
800° C. in a stainless steel retort in an atmosphere com
posed of 5% hydrogen and 95% nitrogen by volume and
was cooled in this atmosphere. The product had an elec~
trical resistivity of 7.8)(102 ohm-cm. by the DPR test 55
and therefore was rated as satisfactorily conductive.
Example II
When a portion of the same aluminum chloride-con
taining zinc oxide used in Example I was similarly proc
ment in the reducing atmosphere.
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,147,379
2,585,461
Pearlman ____________ __ Feb. 14, 1939
Hirsch ______________ __ Feb. 12. 1952
2,887,632
Dalton ______________ __ May 19, 1959